We took our sons (ages 3 & 7) to this zoo and were all thrilled with the experience.  This zoo is free allowing us to re-enter at our leisure.  Not only was it free, but the entire zoo was wonderful.  All animals were active and easy to spot, even if they were resting.  Most zoos that we have been to the animals are inactive or hard to see.  Tip:  Before planning your trip to the zoo, try coordinating with the Chicago Water & Air show.  This was awesome because while we were in the zoo we could also watch all of the planes above.